Each of the referenced prior patents discloses a two-part jack system which includes a mechanical jack stand that is raised or lowered by a detachable power unit. In the particular system shown in U.S. Pat. No. 4,462,569, the power unit is also utilized for transporting two or more jack stands. In a system of that type the cooperation required between the two units of the system has many different aspects, including:
(1) Picking up a jack stand for purpose of transport. PA1 (2) Transporting a jack stand and positioning it where needed. PA1 (3) After the jack stand is positioned, utilizing it to raise a load. PA1 (4) Detaching the power unit from the raised, loaded jack stand without disturbing the load. PA1 (5) Moving a second jack stand, already carried by the power unit, into a position of readiness for use. PA1 (6) Re-attaching the power unit to a raised, loaded jack stand. PA1 (7) Operating the power unit to lower a raised, loaded jack stand in a safe manner. PA1 (8) Transporting a lowered, unloaded jack stand to a different location.
As shown in each of the referenced previous patents, the mechanical jack stand includes a pair of telescoping vertical frame members, and the lower and stationary frame member carries a spring-loaded pawl while the upper and raisable frame member carries a series of ratchet teeth adapted to be engaged by the pawl. The pawl has a downwardly extending release arm whose lower extremity may be thrust or pivoted towards the frame members for disengaging the pawl from a particular ratchet tooth with which it is then engaged.
When the jack stand is under load, the vertical load on the ratchet tooth and pawl prevents the pawl arm from being easily disengaged. In accomplishing step (7), above, it is therefore desirable, and in fact usually necessary, to first operate the power unit for lifting the load, at least momentarily, so that the pawl can be disengaged. While that lifting action is being maintained, an inward thrust is then applied to the lower extremity of the pawl release arm for releasing the pawl.
As disclosed in U.S. Pat. No. 4,697,788, the inward thrust is preferably applied by a ratchet release member (such as a spring-loaded pusher) whose operation is normally restrained except when the jack stand is to be lowered. Removal of the restraint then permits the spring-loaded pawl to disengage from the ratchet tooth as soon as the vertical load is lifted from the tooth.